Simulating global ice volume across the Mid-Pleistocene Transition with a ramp-like increase in the deglaciation threshold

Abstract. During the Quaternary period, spanning the last 2.6 million years, the characteristic frequency and amplitude of glacial-interglacial cycles evolved from low-amplitude 41 000-year cycles to high-amplitude 100 000-year cycles. This transition occurred around 1.2 to 0.8 million years ago and is referred to as the Mid-Pleistocene Transition (MPT). The absence of any significant change in the external orbital forcing during this period suggests the existence of some fundamental change within the Earth-climate system, leading to non-linearities or feedback mechanisms. The temporal structure of such a change is still under debate. Here, we present a new conceptual model of the Quaternary global climate, the so-called RAMP model, which can reconstruct the past global ice volume. The RAMP model incorporates a ramp-like change in its deglaciation threshold to reconstruct the MPT. Parameter optimization finds that the onset of the change occurs in the early Quaternary (2.6–2.3 Ma) and lasts into the 100 kyr world (550–250 ka). These findings support the idea of a long-term shift as a cause of the MPT and imply that the climate shift began early in the Quaternary. The model uses a linear combination of the precession parameter and obliquity as external orbital forcing, which can be optimized to best reconstruct the target curve. The identified orbital forcing differs from the widely used insolation at summer solstice at 65° N, as the latter exhibits a larger precession signal. The RAMP model yields consistent and good results for two different global mean sea level curves. Moreover, we conduct a series of sensitivity tests, in which the RAMP model demonstrates strong robustness. In particular, the identified long-lasting period for the ramp is a very robust feature of the model, underscoring its key role in reconstructing the MPT.